Page 109 - Visions of the Future Chemistry and Life Science

98 A. R. HEMSLEY AND P. C. GRIFFITHS

occur at the colloidal dimension. Given this connection, it is surprising
that there are few studies attempting to correlate architecture and colloid
chemistry.
Proteins are not the only structures within cells to adopt a particular
form dependent upon the intrinsic characteristics of their components.
Self-assembly has been demonstrated in microtubules; cell components
built from proteins that act like tug boats and guide large components to
the interaction sites. Their various conformations are a result of concen-
tration specific self-assembly processes. Similarly, the form taken by mem-
branes is governed by the concentration of the components, the nature of
the surrounding fluids, and physical parameters such as temperature. The
formation of periodic minimal surfaces and other bicontinuous structures
may be an inherent consequence, as seen in the prolamellar bodies of
chloroplasts in plants. In both cases, the genetic code need not define all
possible conformations, merely the required concentration of the compo-
nents in order to initiate the ‘desired’ structure. It is perhaps noteworthy
that the formation of complex membrane systems, and indeed the posi-
tioning of the structural units, is often aided by microtubules presenting
clear evidence of a hierarchy of developmental self-organisation and
assembly.
Microorganisms may produce complex microscopic architecture
involving inorganic components. Common amongst these additions are
calcium and silica. Small, golden-brown algae produce surface discs of
calcium carbonate (coccoliths) which can resemble miniature car hub caps.
These structures, although small, are the principal component of the
White Cliffs of Dover, having accumulated for millennia upon a
Cretaceous sea bed. The siliceous frustules (shells) of diatoms (Figure
6.1(a)) enclose the single-celled alga in much the same way as a petri dish;
one larger half, overlapping the edges of the smaller. Like the coccoliths,
and many other microstructures, these shells are composed of networks of
bars, ridges, pores and spines. Siliceous architecture also occurs on the
surface of some higher plant spores (Figure 6.2(a)) and has been shown to
have a colloidal origin.

6.1.3 Mimicking and modelling nature
The production of artificial microscopic structures with similar architec-
ture to that produced by microorganisms has been pioneered by Stephen
Mann. As in our experiments (below), the production of microstructure